This invention relates to severing and sealing thermoplastic material and, more particularly, to apparatus for using a hot wire to sever a plurality of plies of the thermoplastic material and sealing the severed margins thereof to form bags or similar containers.
In the process of manufacturing thermoplastic bags or other containers, a continuous web of the thermoplastic material is supplied from which individual bags are severed. The severed margins are sealed simultaneously to form the side seams of the bag. Typically, a hot-wire is used to sever and seal the thermoplastic material. The wire is formed of a metal alloy and may have a generally circular, elliptical, or rectangular cross-section and is usually heated to a temperature above about 1000.degree. F. to sever and seal the thermoplastic material, such as a polyethylene resin.
A significant problem with using a hot-wire element is that the heat from the wire causes gaseous polymer degradation products such as aldehydes to form during the severing and sealing operation. Some of these gaseous degradation products become entrapped on the surface or in the bags which are formed. The presence of such gaseous degradation products can produce an unpleasant odor and impart an off-taste to foods later stored in the bag. Past approaches to the problem have focused on accelerating the dissipation of the degradation products from the bag after the bag has been formed, rather than on reducing their creation during bag formation. For example, fans have been used to ventilate the bags while moving along the assembly line after formation and while being packaged into cases. Inventory hold-times have been established to allow time for the degradation products to dissipate from the bags while being stored. Additionally, the bags sometimes are stored in curing rooms heated to above 115.degree. F. to reduce the inventory holding-times.
Nevertheless, odor and taste problems associated with severing and sealing with a hot wire can be a serious problem. Accordingly, there is a need for apparatus that alleviates the odor and taste problems associated with using a hot wire to sever and seal thermoplastic material.